In general, simple punching operations involve placing a sheet of material between a die and punch set. When a force is exerted on the punch which is large enough to drive the punch through the material and into the die, a slug of material is sheered out of the material that corresponds to the shape of the punch and the die. In commercial operations a press machine will be used to drive the punch in a downstroke/upstroke motion in order to move the punch into and out of the material (usually a large sheet or rolled strip).
In a particular stamping operation, a floating stripper can be used. In these applications the floating stripper will descend onto the material first and hold the material in place, deriving the pressure required from compression springs in the die. With the material securely held in position, the punch descends and at some point, the face of the punch will impinge on the oil film provided on surface of the material, if such is present, or on the material itself. The punch will continue to exert pressure normal to the surface, until the metal yields and shears out.
The rate of descent of the punch is controlled by the kinematics of the press, until such time when the punch comes in contact with the strip or oil film. As this occurs, the punch will exhibit some buckling property, particularly if the punch is of a slender configuration. Such buckling is limited by the amount of space between the punch, the stripper plate or any other guiding mechanisms, such as for example, interlocking split bushings, which are commercially available. While such buckling occurs, the ram of the press and the die set continue the downward motion according to its kinematics. The movement of the punch is retarded due to buckling and like a spring, stores energy. When the strength of the material is finally exceeded and the material is sheared out, the slug or the part of the metal directly in front of the punch will shear and fracture out of the strip and move downward through the opening in the die. When this has occurred, the stored energy in the punch is released and the punch is almost instantly reverted to its original full length. The energy released by this sudden acceleration of the mass of the punch may be so high as to exceed the strength of the punch's head and tensile fracture occurs at the transition of the punch body to the punch head. This is a common problem, particularly when punching holes in difficult materials, such as printed circuit fiberglass panels. Having a punch made of massive cross section with only the tip of the correct dimensions is not always possible because of the required proximity of adjacent holes.
The forces between the face of the punch and the surface of the metal can be extremely high. In the absence of lubricants, one will frequently observe the transfer of the image or surface finish of the punch onto the strips, which means that the elastic limit of the material locally has been substantially exceeded. If a lubricant is used, and particularly at higher punch speeds, such oils do not have sufficient time to escape laterally, i.e. its displacement time constant has been exceeded, and the image transfer will not occur. In such cases, the effective fit between the surface of the metal and the face of the punch is such that if the punch is suddenly retracted, a vacuum will exist between the face of the punch and the slug, and if the friction of the slug against the die is low, the slug may follow the punch during its retraction, in effect creating a condition generally referred to as "slug back out". This is an unwanted result, since the slug now is trapped in the tool, which causes damage to the parts being punched and may lead to breakage of the tools.
When fracturing of the metal during punching occurs, there is a byproduct of extremely small particles which accumulate in the oil. These are readily observable by their coloring of the residual oil, according to their substance. In some cases, these particles are trapped between the stripper plate and the strip, and if the pressure is sufficient, mar the surface of the strip which then requires a correction measure such as continuous flushing by a lubricant in the die.